This invention generally relates to the field of electronic devices that incorporate an accessory control bus interface and, more particularly, to a method and apparatus for operating these devices with various types of external accessories, including accessories that do not include compatible bus interfaces.
In order to add complementary features and functionality, many consumer, telecommunication and industrial electronic devices interface with external accessories through suitable accessory ports. When the accessory device and the electronic device are connected, their complementary accessory connectors engage to form corresponding electrical interconnections for accessory signals. For example, a portable wireless phone can be attached to a suitably formed cradle of a hands-free external accessory, to facilitate the operation of the phone from inside an automobile. Usually, the mobile hands-free device includes audio circuitry for amplifying the audio heard in the automobile and other circuitry for routing voice signals from the automobile into the phone. Another version of the hands-free external accessory is a less complicated portable hands-free accessory that includes a small speaker and a microphone. When the portable hands-free accessory is worn by the user, the speaker and microphone are positioned close to the user's ear and mouth, respectively. In this way, portable hands-free accessory allows a user to operate the phone without holding it in his or her hands. Another popular accessory for the phone is a battery charger, which may be incorporated into the mobile hands-free accessory, to charge the phone's battery.
When operating with an external accessory, the electronic device may be required to adjust its operating parameters. For example, when operating with the portable hands-free accessory, the phone increases the audio output power provided at the accessory port, to accommodate the portable hands-free accessory. Accordingly, most electronic devices are equipped with sensing circuitry that sense presence or absence of the external accessories.
Many conventional sensing circuits of electronic devices sense a predefined electrical condition that is created on the accessory port, when the external accessory is present. Some external accessories present a predefined binary state (a high or a low state) to indicate their presence. Alternatively, external accessories may present an electrical parameter, such as an impedance, on the accessory port, that enables the electronic device to distinguish between various types of external accessories. By sensing a particular impedance, which may be represented by a voltage potential at the accessory port, the electronic device can determine the type of the attached external accessory.
With the advances in technology, the electronic devices and their accessories are becoming more and more complicated. Often, the more advanced electronic devices incorporate intelligent micro-controllers for controlling various functions such as driving displays and input/output ports, etc. To keep up with these advances, the manufacturers are equipping the external accessories with intelligent controllers as well. Consequently, there has been a need to communicate more complicated operating parameters between the electronic device and their external accessories. In these cases, the operation of the electronic device and the external accessory may require the repeated communication of one or more dynamically changing operating parameters based on which the operation of the electronic device and/or its accessory may be adjusted, to achieve or improve a particular function. For example, when crossing from one communication cell into another, the phone may transmit to the mobile hands-free external accessory a hand-over parameter along with audio parameters that correspond to the operating environment of the new cell. By taking into account the operating parameters of the new cell, the mobile hands-free device can adjust the parameters of its echo-canceling circuitry, to provide better audio quality inside the automobile.
Some of the more advanced electronic devices and accessories use an accessory control bus (ACB) for transferring operating parameters with each other. One such ACB interface is the I.sup.2 C-bus interface, which is developed by Phillips Corp. The specification and functionality of the I.sup.2 C-Bus interface is described in detail in a Phillips Semiconductors' publication, titled: The I.sup.2 C-bus and how to use it (including specifications), April 1995, which is hereby incorporated by reference. Over a two-line serial bus, the ACB devices that incorporate the I.sup.2 C interface can transfer a wide variety of complex operating parameters at rates of up to 400 kbits/sec.
Often times, however, it is required to provide interoperablity between ACB devices and non-ACB devices. For example, an ACB device, which is equipped with the I.sup.2 C bus interface, is sometimes required to operate with a simple non-ACB accessory that does not incorporate the I.sup.2 C interface. Conversely, an ACB accessory equipped with the I.sup.2 C functionality may be required to interface with a non-ACB device. Conventionally, approaches to interoperability involve complex modifications to the ACB, or the non-ACB devices. These modifications often increase the cost of providing the interoperability. Therefore, there exists a need for a simple method and apparatus for providing interoperability between the ACB and non-ACB devices.